Tech question about sensor size and magnification

[MisplacedAngler]

If I'm looking at buying a lens that has a magnification of .25 on a full frame sensor, the magnification on a APS-c size sensor would be ~.5 correct?

If not can someone straighten out my thinking

 

[Overread16 4]

Magnification ratios are a measurement based on the size of the reflection made on the sensor by the lens. Thus changing the size of the sensor won't change the magnification that the image that is reflected onto it. It will mean that the image, of the same magnification, will have the edges cropped away from it when used on a crop sensor. This is important because it means if you then print a photo from the fullframe and the crop sensor to the same dimensions (eg 8*10) then the image from the crop sensor will appear have been taken at a greater magnification because whilst the edges were cut away the resulting image is stretched over a larger canvas size.

Out of interest how come the magnification of 0.25 is important? Is it for measurements?